The invention especially relates to a means for controlling a coil arrangement with variable inductance which allows the inductance to be varied at particularly rapid rates. This means can then be used whenever the inductance of the coil arrangement is varied by means of current-induced pre-magnetization and when at least two working windings are provided which are connected in parallel.
The invention can basically be employed in all applications in which current-controlled, variable inductors are needed to control an electric alternating current. More specifically, the invention can be applied to current-controlled, variable inductors having a control winding and two working windings connected in parallel as shown, for example, in FIG. 2.
Coil arrangements with variable inductance are used in power engineering and telecommunications applications. One invention-related application of coils with variable inductance is in the area of switching power supplies in order to adapt the energy taking place in the high-frequency range to changing load requirements.
Examples of such switching power supplies are described in “High Power Densities at High Power Levels” by A. Jansen et al. in CIPS 2002, 2nd International Conference on Integrated Power Systems, 11-12 Jun. 2002, Bremen, Germany and in German Patent Application 103 21 234.5, to which reference is made.
To realize such electrically controlled inductance, the effect in which the relative magnetic permeability of ferro and ferromagnetic materials decreases together with the magnetic flux density in the material can be exploited. Based on this principle, numerous coil arrangements have been proposed in the past which, by means of a current in a control coil, cause a magnetically highly permeable coil core to be pre-magnetized and in this way control the inductance of the inductor winding, also positioned on the coil core.
U.S. Pat. No. 6,317,021 proposes that two inductor windings be connected in parallel in such a way that the magnetic fluxes for the control winding generated by these windings cancel each other out.
German Patent Application 102 60 246.8 proposes a coil arrangement with variable inductance having two separate toroid coils which carry inductor windings, as well as a control winding which encompasses the two wound toroid coils in order to pre-magnetize the core material of the toroid coils.
The invention can particularly be applied to current-controlled, variable inductors which have inductor windings connected in parallel as outlined above in reference to U.S. Pat. No. 6,317,021. In the following description, the term inductance thus refers to the inductor or working windings, and particularly to the working windings connected in parallel, of such a coil.
In such coil arrangements, a direct current in the control winding brings about DC pre-magnetization of the entire core material and thus changes the inductance of the working windings. It is clear that the direction of the direct current for pre-magnetization is arbitrary.
The main disadvantage of these current-controlled, variable inductors is the relatively long demagnetization time of the core material resulting in a slow change in inductance from lower to higher inductance. If the variable inductor is used, for example, as an AC power valve in the secondary regulation loop of a switching power supply, this sluggishness results in considerable overshoot once load jumps that go from a high load to a low load appear. This voltage overshoot is countered in the prior art by clamping circuits. These clamping circuits, however, expose a large number of components to high stress due to short-circuit currents.
In the past, the problem thus arose that the current-controlled, variable inductors of the type described could only run through inductance variations very slowly, that is they could only vary their inductance from a minimum value to a maximum value within several milliseconds.
It is therefore the object of the invention to accelerate this process of inductance variation and accordingly to make damper circuits superfluous and to prevent the high component stress associated with them.